Several pathogens such as baculovirus, Listeria, and Shigella hijack their host cell’s own actin cytoskeleton in order to propel themselves into other cells and spread infection. Behind each pathogen is a comet made of actin filaments and associated actin regulators, but the mechanism of propulsion and the structure of the actin comet have been debated. A recent paper in PLoS Biology by Mueller and colleagues describes the use of electron tomography to show a fishbone-like array of actin filaments behind baculovirus, the smallest pathogen known to use propulsive actin comets. These comets use an average of four actin filaments at any one time to propel the virus. Using these results, the researchers ran computer simulations that support a model of propulsion in which actin filaments are continuously tethered to the pathogen. The image above shows a negatively-stained actin comet tail behind a baculovirus particle (BV), and the 3D projection of the image shows branch points of the actin tail as red dots. Insets in top image show details of the branch points, and grey tube is a microtubule.
BONUS! Below is a movie of baculovirus propelling itself around a cell. Virus particles are red, and actin comet tails can be seen in green behind the particles.
Mueller J, Pfanzelter J, Winkler C, Narita A, Le Clainche C, et al. (2014) Electron Tomography and Simulation of Baculovirus Actin Comet Tails Support a Tethered Filament Model of Pathogen Propulsion. PLoS Biol 12(1): e1001765. doi:10.1371/journal.pbio.1001765.